This invention relates to a method and apparatus for preventing the copying of a video program. More particularly, it relates to detecting, a modified color burst signal in even a single television line of a video signal to cause disabling of the recording portion of a video recording device when the modified video signal is detected.
Various apparatuses and methods have been developed for modifying a video signal so that a normal color picture may be produced by a television receiver receiving the modified video signal, but that video recording of the modified video signal is impaired. That is, a video picture produced from the video signal as recorded on a video tape recorder is altered so that it makes unacceptable viewing.
U.S. Pat. No. 4,631,603 (""603) entitled xe2x80x9cMethod and Apparatus for Processing a Video Signal so as to Prohibit the Making of Acceptable Video Tape Recordings Thereof,xe2x80x9d which is incorporated by reference, discloses such a method and apparatus. It describes adding ordered pairs of pseudo-sync pulses and white pulses during the vertical blanking interval. These pulse pairs act in concert to confuse the automatic gain system of a video cassette recorder (VCR), leading to generally unviewable pictures.
U.S. Pat. No. 4,907,093 (""093) entitled xe2x80x9cMethod and Apparatus for Preventing The Copying of a Video Program,xe2x80x9d which is incorporated by reference, discloses a method and apparatus for detecting the ordered pairs of pseudo-sync pulses and white pulses described in the ""603 patent and disabling the recording function of a video cassette recorder. The ""093 patent discloses several detection methods.
U.S. Pat. No. 4,577,216, xe2x80x9cMethod and Apparatus For Processing a Video Signal,xe2x80x9d issued Mar. 18, 1986 and incorporated by reference, discloses modifying a color video signal to inhibit the making of acceptable video recordings thereof A conventional television receiver produces a normal color picture from the modified signal. However, the resultant color picture from a subsequent video tape recording shows variations in the color fidelity that appear as bands or stripes of color error. Colloquially the modifications are called the xe2x80x9cColorstripe(trademark)xe2x80x9d system or the xe2x80x9cColorstripe(trademark)xe2x80x9d process. The term xe2x80x9cColorstripexe2x80x9d is a trademark of the assignee of this application. Commercial embodiments of the teachings of this patent typically limit the number of video lines per field having the induced color error or color stripes.
Color video signals (both in the NTSC and PAL color television standard systems) include what is called a color burst. The Colorstripe(trademark) system modifies the color burst. The suppression of the color subcarrier signal at the television (TV) transmitter requires that the color TV receiver include (in NTSC) a 3.58 MHz oscillator which is used during demodulation to reinsert the color subcarrier signal and restore the color signal to its original form. Both the frequency and phase of this reinserted subcarrier signal are critical for color reproduction. Therefore, it is necessary to synchronize the color TV receiver""s local 3.58 MHz oscillator so that its frequency and phase are in step with the subcarrier signal at the transmitter.
This synchronization is accomplished by transmitting a small sample of the transmitter""s 3.58 MHz subcarrier signal during the back porch interval of the horizontal blanking pulse. FIG. 1 shows one horizontal blanking interval of an NTSC color signal. The horizontal sync pulse, the front porch and blanking interval duration are essentially the same as that for black and white TV. However, during color TV transmission (both broadcast and cable) 8 to 10 cycles of the 3.58 MHz subcarrier that is to be used as the color sync signal are superimposed on the back porch. This color sync signal is referred to as the xe2x80x9ccolor burstxe2x80x9d or xe2x80x9cburstxe2x80x9d. The color burst peak-to-peak amplitude (40 IRE for NTSC TV as shown) is the same amplitude as the horizontal sync pulse.
FIG. 1A shows an expanded view of a part of the waveform of FIG. 1 including the actual color burst cycles. During the color TV blanking intervals, such a color burst is transmitted following each horizontal sync pulse.
In one commercial embodiment of the Colorstripe(trademark) process, no color burst phase (stripe) modification appears in the video lines that have a color burst signal during the vertical A blanking interval. These are lines 10 to 21 in an NTSC signal and corresponding lines in a PAL signal. The color burst modifications occur in bands of four to five video lines of the viewable TV field followed by bands of eight to ten video lines without the color burst modification. The location of the bands is fixed (xe2x80x9cstationaryxe2x80x9d) field-to-field. This Colorstripe(trademark) process has been found to be quite effective for cable television, especially when combined with the teachings of U.S. Pat. No. 4,631,603 also invented by John O. Ryan and incorporated herein by reference.
In NTSC TV, the start of color burst is defined by the zero-crossing (positive or negative slope) that precedes the first half cycle of subcarrier (color burst) that is 50% or greater of the color burst amplitude. It is to be understood that the Colorstripe(trademark) process shifts the phase of the color burst cycles relative to their nominal (correct) position, illustrated for example in FIG. 1A. An example of a phase shifted color burst is shown in FIG. 1B. The amount of phase shift shown in FIG. 1B is 180xc2x0 (the maximum possible) over the entire color burst.
Further, the amount of phase shift in the Colorstripe(trademark) process can vary from e.g. 20xc2x0 to 180xc2x0; the more phase shift, the greater the visual effect in terms of color shift. In a Colorstripe(trademark) process for PAL TV, a somewhat greater phase shift (e.g. 40xc2x0 to 180xc2x0) is used to be effective.
FIG. 1C illustrates a color burst signal which is partially modified, that is, is partially phase shifted, to prevent making a recording of the associated video signal. In an exemplary NTSC embodiment, such as disclosed in the PCT Application Number US96/17719 incorporated by reference, a system called Advanced Switched Burst (ASB) has the capability of programming various combinations of partially modified color burst signals. FIG. 1C illustrates one of such combinations, by way of example only, wherein the color burst envelope comprises three zones. Zone 1 (burst start) begins 4.96 microseconds (xcexcsecs) after the leading edge of horizontal sync. Zone 1 ends 1.48 xcexcsecs after normal burst start. Zone 2 begins and ends at the end of Zone 1. In this particular embodiment, there is no Zone 2 per se. Zone 3 begins after the Zone 2 point and extends 1.48 xcexcsecs to the end of burst. Therefore, in this particular embodiment, the color burst has a width of 4.96 xcexcsecs. The Zone 1 area will contain modified (inverted 180xc2x0) subcarrier. Normal phase subcarrier is used in Zone 2 (zero duration) and Zone 3.
Although three zones are illustrated in FIG. 1C by way of example, the color burst may be partially phase shifted using other numbers of zones, e.g. two, etc., and phase shifting may be applied to various ones of the zones.
Other variations of the Colorstripe(trademark) process are described in the following provisional applications: Serial No. 60/010015 by William J. Wrobleski entitled An Improved Method And Apparatus For Modifying The Color Burst To Prohibit Video Tape Recording, Serial No. 60/010779 by William J. Wrobleski, entitled An Advanced Color Burst Method And Apparatus For Modifying The Color Burst To Prohibit Videotape Recording, Serial No. 60/014246 by William J. Wrobleski, entitled A Method And System Of Copy Protection Using An Advanced And Split Color Burst. The three applications were combined in a U.S. non-provisional patent application which issued Feb. 4, 2003 as U.S. Pat. No. 6,516,132.
Methods and apparatuses for defeating the effects of the color burst modifications of the ""216 patent and others are described in U.S. Pat. No. 4,626,890 entitled xe2x80x9cMethod and Apparatus for Removing Phase Modulation From the Color Burst,xe2x80x9d and incorporated by reference. Methods and apparatuses for detecting a Colorstripe(trademark) signal and for additional defeat methods and apparatuses are described in application Ser. No. 08/438,155 by Ronald Quan and John O. Ryan, filed May 9, 1995 entitled Method And Apparatus For Defeating Effects Of Color Burst Modifications To A Video Signal, U.S. Pat. No. 5,784,523 granted Jul. 21, 1998, U.S. Pat. No. 6,516,132 granted Feb. 4, 2003 and U.S. Pat. No. 6,327,422 granted Dec. 4, 2001 are incorporated herein by reference.
The present invention provides an apparatus and method for preventing the recording of selected video signals. More particularly, the present invention provides for detecting a Colorstripe(trademark) modified video signal at a recording device, and disabling the recording device when the respective modified color burst signal is detected. Unmodified signals correspondingly result in the recording device being enabled, so that the video signal may be recorded. However, when video tapes are originally recorded with a recognizable copy protecting modified color burst waveform, the tape recording device is completely disabled so that it cannot record the video signal at all.
More particularly, a video signal is supplied to a recording device and also to a disabling means whose output is a control signal for enabling or disabling the recording device. That is, if the video signal is modified to include a partially or entirely modified, for example, phase shifted, color burst signal on even one line of the video signal, the recording device will be disabled by the disabling means to prevent an acceptable recording of the modified video signal. If no modified color burst signal is detected by the disabling means, an acceptable recording is allowed.
The disabling means includes means for generating a timing window which encompasses the time duration of the back porch interval and thus of the color burst signal. The video signal, modified or unmodified, thus is supplied to a phase lock loop circuit only during the presence of the color burst signal. A color burst signal which is modified by being partially or entirely phase shifted causes a voltage to be produced by the phase lock loop circuit which is different than the voltage produced when an unmodified color burst signal is supplied to the phase lock loop circuit. Every television line is checked to determine if any line is modified. Since the detection process is limited by the window generator to the interval during which only the color burst signal is present, the phase lock loop circuit cannot respond to active video or to other non-color burst signals. The voltage difference between a modified and unmodified television line is sampled and used to respectively disable or enable the recording device.